1. Field of the Invention
The present invention relates principally to electrochemical measurement, and more specifically to the measurement of localized electrochemical corrosion and other heterogeneous electrochemical processes such as cathodic and anodic protection of metals, electroplating, electrotyping, electrometallurgy, electrowinning and electromachining.
2. Description of the Related Art
A metal surface exposed to an electrolyte is often electrochemically non-uniform due to factors such as a localized chemical environment over the metal surface, an inhomogeneous metallurgical structure in the metal surface, localized defects in a surface protective film which covers the metal surface, and a polarization voltage which non-uniformly polarizes the metal surface. This electrochemical non-uniformity is a common natural phenomenon and is referred to as electrochemical heterogeneity. Electrochemical processes occur over a heterogeneous metal surface are referred to as heterogeneous electrochemical processes. Heterogeneous electrochemical processes are characterized by a marked difference in electrochemical parameters, such as electrochemical potential, over the metal surface.
Heterogeneous electrochemical processes are very common in practice and often play a key role in corrosion and electrochemical industries. Localized corrosion such as pitting corrosion and crevice corrosion is probably the most common heterogeneous electrochemical process. Localized corrosion often causes premature, in some cases, catastrophic failure of industrial structures. When localized corrosion occurs, there is a spatial separation of the anodic and cathodic areas. Different electrochemical reactions occur on the anodic and cathodic areas and there is a marked potential difference between anodic and cathodic areas. This electrochemical potential difference drives galvanic current to flow between anodic and cathodic areas simultaneously in the metal and in solution, resulting in rapid localized corrosion penetration to occur at anodic areas. Heterogeneous electrochemical processes also play a key role in cathodic and anodic protection of metals, electroplating, electrotyping, electrometallurgy, electrowinning, electromachining. In the case of cathodic protection of a metal structure using a sacrificial anode, protection current (galvanic current) is not uniformly distributed over the metal structure surface. Locations that are far away from the sacrificial anode site often have low protection current density and thus may not be effectively protected. This is a major problem that has to be addressed when a cathodic protection system is designed. Similar problems arise when impressed cathodic protection current is applied to a metal structure such as a long pipeline with protective current density decaying as the distance to impressed current source increases. In this case, locations far away from the current source may not be effectively protected. Electrochemical heterogeneity is also a key factor for anodic protection. Indeed, anodic protection potential and its distribution is of major concern and it could significantly affect the efficiency and reliability of an anodic protection system. Electrochemical heterogeneity is also common in electroplating and electrotyping. If a work-piece (an electrode) has a complex shape, the distribution of electrochemical reaction current (electroplating or electrotyping current) can be very non-uniform over the surface of the work-piece and this electrochemical heterogeneity can influence the quality of electroplating and electrotyping. In the case of electrometallurgy and electrowinning, electrochemical heterogeneity can result in a non-uniform electrodeposition and can influence the structure of crystals. In an electromachining processes, electrochemical heterogeneity could also affect the precision of electromachining, especially when a work-piece has a complex shape.
The measurement of heterogeneous electrochemical processes is an important requirement for characterizing, monitoring, controlling and optimizing these industrially important processes. However, the measurement of heterogeneous electrochemical processes is a major difficulty in electrochemical and corrosion science and engineering. This is because conventional electrochemical techniques have major limitations in measuring local electrochemical parameters and in determining local electrochemical kinetics. It is well known that conventional electrochemical techniques use a one-piece metal electrode which only measures mixed and averaged electrochemical parameters over the whole electrode surface. When a one-piece electrode is used, it is impossible to measure the galvanic current that flows between localized anodic and cathodic sites in the electrode body since an ammeter is not able to be inserted between anodic and cathodic sites which are located on a single piece of metal surface. It is also well known that traditional electrochemical techniques have major difficulties in determining heterogeneous electrochemical kinetics. This is because traditional electrochemical kinetic theories are based on a one-piece electrode with an ideally uniform working surface. The fundamental formulation describing the electrochemical kinetics over the metal surface, the Butler-Volmer equation, is based on a uniform electrochemical corrosion mechanism. Traditional electrochemical techniques which are based on the Butler-Volmer equation such as the Tafel polarization technique, the linear polarization technique and the AC impedance spectroscopy, in principle, are applicable only to the measurement of the electrochemical kinetics of a uniform electrode surface, for instance, to the measurement of rates of uniform corrosion.
The fundamental limitation associated with conventional electrochemical measurement has not been overcome in despite of extensive research in this area over the past several decades. The scanning reference electrode technique and the scanning vibrating electrode technique have been used to estimate and to map current flows in the electrolyte phase, however these techniques only measure the currents in solution and not at the surface of the metal and thus the scanning results from these techniques do not delineate clearly the areas of cathodes and anodes, see H. S. Isaacs et al., "Mapping Currents at the Corroding Surface/Solution Interface", Proceedings of Research Topic Symposium, Corrosion 97, NACE, 1997, p. 65. Electrochemical impedance spectroscopy has been proposed to monitor localized corrosion, see F. Mansfeld et al., "Monitoring of Localized Corrosion with Electrochemical Impedance Spectroscopy", Corrosion 92, NACE, 1992, paper 229, however this techniques still use a conventional one piece electrode and thus it may suffer similar limitations as other conventional electrochemical methods in studying localized corrosion. Electrochemical noise analysis has also been proposed to monitor localized corrosion, see K. Haldky, "Corrosion Monitoring", U.S. Pat. No. 4,575,678 issued Mar. 11, 1986 and D. A. Eden et al., "Corrosion Monitoring", U.S. Pat. No. 5,139,627 issued Aug. 18, 1992, however this application is under development and remains controversial, see F. Mansfeld et al., "Comments", Journal of the Electrochemical Society, vol. 141, 1994, p1403. A corrosion sensor array was used to instantaneously monitor corrosion and environmental, see R. S. Glass et al., "Method for Monitoring Environmental and Corrosion", U.S. Pat. No. 5,437,773 issued Aug. 1, 1995, however that sensor appears only carrying conventional electrochemical measurements at selected isolated locations.
The present invention provides an electrochemical measurement method utilizing a unique integrated multi-sensor electrode system, namely the wire beam electrode, for measuring localized corrosion and other heterogeneous electrochemical processes. This method overcomes some major limitations associated with conventional electrochemical measurement techniques and it can be used to measure local electrochemical parameters which are important for controlling and optimizing industrial electrochemical processes. This method provides a means which is capable of detecting catastrophic localized corrosion and which is capable of monitoring heterogeneous industrial electrochemical processes such as cathodic and anodic protection of metals, electroplating, electrotyping, electrometallurgy, electrowinning and electromachining.